Apparatus for photoelectric detection



March 10, 1953 s. M. BAGNO 2,631,273

` APPARATUS FOR PHOTOELECTRIC DETECTION Filed June 29, 194e 2 sHEETs-sHEET 1 4 i AA AAA www A Trae/VE?? 2 SHEETS- SHEET 2 S. M. BAGNO APPARATUS FOR PHOTOELECTRIC DETECTION HHHH'H March l0, 1953 Filed June 29, 1948 Patented Mar. 10, 1953 APPARATUS FOR PHOTOELECTRIC DETECTIGN Samuel M. Bagno, Astoria, N. Y., assignor to Thev Alex-tronic Protective Corporation of America, New York, N. Y., a corporation of New YorkY Application June 29, 1948, Serial-No, 35,867

19 Claims. 1

The present invent-ion relates to a method and apparatus for photoelectric detection and in parfticular to one adapted to controla -burglaror intruder alarm.

It has long been proposed to employ photoelectric detection apparatus as a burglar alarm, said apparatus comprising a beam of light directed onto a suitable photoelectric element so that whenever said beam of light is interrupted, as by the passage of a person thereacross, the energization of the photoelectric element will be so changed as to set off an alarm. Two primary defects have been inherent in such systems, have mitigated'strongly against their use, and have prevented their being approved for local alarm workv by insurance companies and protective agencies. These defects are: (1) the susceptibility of the apparatus to counterfeiting, thus negativing its usefulness -as a positive intruder alarm, and (2) the high power requirements of the source of illumination, thus making impossible employment of a practical source of `standby power.

Counterfeiting or "fooling of a photoelectric detection system of thenormal type is relatively easy `provided that the would-be intruder knows of its existence andof the location ofthe photoelectric celL All thathe need-ido in order toef.- fect his undetected entranceinto.. the premises .guar-:ledby the system is to .direct a beam oilight upon the photoelectrc cell at least-forA the pe.- riod of time during which he himself. is interrupting the genuine light beam. The int-rucler counterfeit` light beam will prevent the alarm from sounding and consequently will render the cient electrical energy to illuminate a lightsource so that it is-capable oi `functioning.in the detec-y tion system,v astandby source of electrical. power would have to be of such magnitude Las to be wholly impractical,- since it is necessary that the standby rSource of power becapable of .energizing i 2A and illuminating the apparatus over an extended period of time.

Itis the prime object of the present invention to .devise a method and apparatus for photoelec-` tric detection which avoids the above disadvantages.

In particular, it is an important object of the present invention to devise such a methodand apparatus which is incapable of being foole.d, that, is to say, which will not be renderedjn capable of setting off the alarm by. reason `of any actions taken by the wouldfbe intruder. Another equally important object of the present invention is to devise a method and apparatus for photoelectric detection which is so constructed as to be capable of being energized by a standby power source comprising standard batteries.

A subsidiary object of the present invention is tc devise anelectronic control circuit for use in a photoelectric detection apparatus which is `Simple, dependable and fool-proof.

Yet another object of the present invention is to devise a simple and eicient power circuit arrangement by means .of which a regular source of power and a standby source of power are both connectable to the apparatus and effective to energize it under appropriate conditions.

A still further object of the present invention is to-devise various circuit subassemblies which are particularly effective in the practice of the novel method herein disclosed.

The above objects are achieved by utilizing a lightzsource of high intensity, and of illuminating thatlight sourcecin an intermittentmanner, thus reducing the power drain on the standby-power source. The control circuits are so arranged that only when the lflashes of light received by the photoelectric element are in exact synchronism with the flashes of light emitted by the source `of illumination will the alarm be restrained.

Such synchronism involves a correspondence not only of frequency but also of phase so that it is for all practical purposes impossible for a wouldbe intruder to simulate with an auxiliary source of illumination the exact flashes of the true source of illumination, and this evenvif he knew and were able accurately to reproduce the frequen-cy of said flashes.

To the accomplishmentof the foregoing objects and such other objects as may hereinafter appear. the present invention relates to the method, apparatus and circuit arrangement of a photoelectric detection system as deiined in the appended claims and as described-'in this'specication, taken together with the accompanying drawings, in which:

Fig. l is a schematic representation of a room showing the system in operation and illustrating the manner in which the system would detect the presence of an intruder;

Fig. 2 is a circuit diagram disclosing the details of the apparatus; and

Fig. 3 is a circuit diagram illustrating the power supply for the system.

Broadly considered, the apparatus of the present invention comprises a light source generally designated A and a light sensitive element such as a photoelectric cell generally designated B, the light source being illuminated and the light emanating therefrom being directed to the photoelectric cell B so as to energize the same. As illustrated in Fig. l the light source A may be positioned in a room generally designated 2 so `that the light emanating therefrom generally designated d passes across the entrance to the room, here shown as the door G, it being understood that a window or any other entranceway could be equally protected, so that if the door 6 should open, or if an intruder should pass therethrough, the light beam d would necessarily be interrupted, thus interrupting the energization of the photoelectrie cell B and setting off a sensible alarm of any desired type. These elements and arrangements Vare common to most photoelectric detection systems.

An important novel element in my apparatus is a means generally designated C which causes the light source A to iiash intermittently so that the light beam 4 is not a continuous beam but is instead made up of a series of discrete light pulses indicated schematically in Fig. l by the curved lines 8. It is preferable, but not necessary, that the rate of flashing of the light source A be so rapid as to be greater than the persistence of the human eye. It is well known that light ashes at a frequency of 60 per second are undetectable by eye and in a preferred modification the ashes may occur on the order of 200 times per second.

The frequency of flashing has two significant effects. In the first place, if the would-be intruder does not know that the light source A is only intermittently energized, he will be under the impression that it is a continuous source of illumination rather than an intermittent source and should he attempt to fool the device with a counterfeit source of illumination of uninterrupted intensity, he would succeed only in setting off the alarm.

In the second place, even if the intruder knows that the light source A is intermittent 1n character, he would still, in an attempt to fool the apparatus, be required to utilize a counterfeit light source of exactly the same frequency, and if the frequency is so great as to be undetectable to his eye, he will not be able accurately to determine its frequency without the assistance of relatively complex auxiliary apparatus such as a stroboscope. The greater the frequency of the light flashes from the light source A, the more diicult would it be for the intruder to ascertain their frequency and to generate simultaneous dashes. As will be explained in detail hereafter, unless the dashes were simultaneous the alarm would be set oir.

The light sensitive element B is connected to a suitable electrical circuit for detecting its changes in output, which changes in output will be determined by the frequency and intensity of the light dashes impinging thereupon and which will be converted into rst electrical signals. The output of the light sensitive element B will, of course, be modified by ambient light and random noise effects, but the only useful p-art of the first electrical signals will be the output caused by the received iiashes from the light source A. A signal circuit generally designated D is connected to the light source A so as to generate second electrical signals in exact synchronism with the flashes of the light emanated by the light source A. A control element generally designated E is provided for comparing the first and second signals, the control element actuating an alarm control circuit generally designated F, the control element E and the alarm control circuit F cooperating so as to set oii a sensible alarm whenever the first and second signals are not in exact synchronism both as to frequency and phase. Since the first electrical signal may and probably will have components other than those generated by the flashes of light from the light source A, it therefore is not necessary that all of the first signal be in exact synchronism with all'of the second signal. It is necessary only that the two signals both contain substantial components in synchronism, and throughout this specication the term synchronism is meant to refer to the situation where substantial components of both signals are in synchronism. The fact that the system operates only on these substantial components makes it independent of ambient light and of random noise effects, thus increasing the dependability and sensitivity of the system. It also seems appropriate at this point to differentiate the present system over others known to the prior art Where modulated light is employed. In such prior art systems, the frequency of modulation is a controlling factor and tuned circuits are employed to detect those modulations. In the instant system, the term frequency does not refer to modulation at all but refers instead to the number of discrete flashes occurring per unit time from the light lsource A. Hence tuned circuits are ineffective and the circuit arrangement disclosed hereinafter is employed.

Hence, even if the would-be intruder were able to determine the exact frequency of the ashes of the light source A, as by means of a stroboscope, and were able exactly to reproduce that frequency of flash in a counterfeit light source, as might-be possible with certain commercial and laboratory equipment, he would still be unable to fool the device because of his inability, from a practical point of view, to secure exact simultaneity between the pulses from the counterfeit source and the true source A. Extensive experiments have been conducted by me with highly precise auxiliary light sources tuned to ash at exactly the same speed as the true light source A and every one of those experiments, carried out with the greatest zeal, have proven unable to fool the apparatus.

A power supply is provided for the apparatus and is illustrated in Fig. 3, this power supply including a regular source of power G, a standby source of power H, and a switch I for cutting in the standby source of power H when the regular source of power G falls below a predetermined minimum. By reason of the fact that the light source A ashes only intermittently, the power drain on the power supply is quite minimal and consequently it is possible to form the standby source H of a suitable number of standard batteries, preferably of the dry cell or A and B bat- 5 tery type; lthese batteries being 'capableof 'encrev gizin'g the apparatus -overa iong period of time;

The -apparatus is sof constructed y that if "both: powersupplies should fail-at. any ftirne, thev alarm will be sounded,"thuseliminating the. pos sibility that one might be depending `uporiza detection system-which was 4wholly. ineffective by reason of y power .f failure;

Fig. 2 disclosesvthefdetailsloi:'the apparatus which -permitthe 'attainment of the :above ob- A jectsin the generalized manner just' described. The light source A maybe in the-form of a neon tube .I0 whichwill illuminate only when the po` tential difference impressed across itsielectriodes I2 'and `I4 exceeds a given value Illumination will continue. .so-long as. the .Voltage 'across'; the electrodes I2 and I4 exceeds another andsmaller value,fcalled the. quench'value.; Thel electrode I4 is connected 4to ground and the electrode. I2 is connected by'rneans of'wirev I6, adjustable -rheoe' stat I8, and voltage dropping rheostat' 20,2110 the high VoltagefD. C.xtap. 22. Apcapacitor 24wis connected across' the tubei0 and so long asthe voltage across the. capacitor is Abelow'f-that 'ref' quired for illumination of the` tube; It), the/con'-Y denser will charge,` thus vslowly .drawing rpower from the power supply. As soon asthe potential difference across the capacitor 24 has built up to a value equal to the firing voltage of the .tube It,

theA tube I0 willlbecome "conductive and the capacitor 24 will discharge :through the tube causing lit to illuminate. This dischargeis pracl' tically instantaneous during Wl'iiclfl'the capacitor 2 4. discharges through the tube,- until Vthe `yoltag'e across itis less: than the quench value. At this pointthe tube will darken and theicondenser will once again charge. The value of thecapacitor will determinethe. frequency with whichtheL neon tube I0 will illuminate and, as has alreadybeen stated, Yit Ais preferred. that this frequency be greater than Athat detectable by the eyeand pref.l

erably on the order of magnitudeof ZOO-times-per second. It will be noted that bythis-arrangement energy is stored in the. capacitor 24 overa considerable period'of time and isvexpended-:in an instant.

The intermittent-natureof the illumination of the` neon tube- I0 constitutes. a source. of great saving in power consumption. .If the tube were constantly illuminated,Y power; would have to. .be supplied to it all ofthe time, l watt being-re#` quired when the neon tubel is illuminated. Since theA capacitor 24 takes."- considerablymlonger to charge than to discharge, 'the tube-I0 Yconsumes* power during considerably less than-one percent*y ofthe time the system is in operationfso that the power consumption is cut to considerably lese than one percent of that which Wouldfnormal-ly be required. A practical burglar' alarm insta-lla@` tion has been constructed and teste'dby--me aci-l cording 'to the teachings' of this invention in which the average power drawn by the -source of illumination A is only on the order of 10 m-illi'-` watts. Hence, since the flashes are onlyin finitesimal in duration and occur only over a small portion of the time during Ywhich the .apparatusr is in use, the total power consumptionis quite small. p

The photoelectric cell'B is' positioned 'sorasrto receivel the flashes of light v.8 from the light source A4 and will, lby its very nature, convert those -re-V ceived light flashes into corresponding surges of electrical energy constituting the first electrical signals. These-signals farev then amplii'lecl-fby7 v means of' the -vacuum tubes 264,5 28, 30 and `v32,`

these. tubes; :being connected togetherinponvenetionalfI ampliiica-tion circuits. r While fourr ;stages of 1 preliminary amplification are: fiereA disclosed.y it'fwill be apparent that fthepffnumberrofv 'stages can be Lvaried at will.:

The amplified output; .fromzithe.: tube .132' is coupled -in .Y conventional f, fashion :via frresistor 1.134 and capacitor.;36,.;to the "grid 38-'of the; vacuum tube. 4I) whichis` alsoan amplification tube. 5 VThis tube is so-biasedy as. tozpassno, current, oran eie'ctive amount-.of current, veven when the amv plified -Jirstv electrical signals frointlmaev photoG-r electric cellA Bi are.` f alone f impressed uponz the grid .38.2

Asignal circuit `generally designated D and in.- cluding the capacitor 42 and the resistorwMfis connected betweenthe photo-electric cell Band thegridc'of" another amplifying tube 48 so that.

wheneverv the tube lII! flashes; surgeszconstituting the second electricalasignals'are; applied to; ,the grid .46.` Thetube' 48fis'so biased'tliat when'these second electrical signals: :arefapplied to. thegrid 45'; the .tube -48 will pass currentf'and amplify. The output-ofthe tube 48'is coupled by meansfof connectorv and 'ahighfpass filter :including 582.130 thegrid 38f of the amplifying tube: 40;? The tubewzis so Vbiased that litwiil eitherpassmo currentzor an ineffective amountiof current when the amp-lined second electrical signals from `the tube 4I3farealone' 'applied to thegrid. 38..I

Howevergxthe, biasv of the tube 4D: ist such A'that it1will-pass an effeciveamount of currentl'when the. ampliiie'dfirst` electrical signals coming from thel fourth 1 amplification :stage: 32- and the amplied iisecond electrical `signa-ls coming 4Jfromv the amplii'ierA-By add v'together in synchronismp both as to frequencwand phase; Hence, the tube,

constituting theaiifth amplification stage', willA ,v have anfoutput: only-if: and when the 'irst elecf trical signals generated by the photoelectric cell Bfand,I determined'by 'the frequency and magni tude of the light flashes'. impinging thereupontare in exact' synchronism' Vboth 1 as to frequency vand phase fwiththe llightiiflashes' generated byI the true-'light source "A: vThe tube 4I!` therefore Aconstitutes the :control elementE which compares the rstfand second signals;

The :output: of the', :tube "40 isn fed :'to" the grid Gir of, the Pamplifying tube'aIZ via a-filter'acircuit 64;- which:converts .thepulsednoutput of the tube dit-into sawtooth wave form; The output ofthe tube'l 4U `may i'alsofbe coupled with :the output; of the preceding amplification stages' by Ameansfof resistors 66;?68, TII)l fand I2 for regenerativeam plification: The outputofithe tubeLGZ'feeds in tothe primary 14"-'of a"transformerli `havinga secondary' --with avcenter-tap* 80." The extremities offth-esecondary 18 are connected wia: capacitorsvZfandB4 'to a pair of selenium cell rectiers -86' and 88 connecte'd together Y in'series opposing. The vleft' hand extremity` of .the sec.- ondaryy .18 is connected; -via' ycapacitor 84', to .the lefthand side'.of-the rectifier 88 andfto the D. lC. voltage tap I34. The mid-point' 99. -between the rectiers 86 '-andwtis :connected via. lcapacitor 92 toenemend-cfa coil 94 whichisthe secondary of' 'a transformer 96.11 'I'heotherendof the sec-f ondary coil '94 is connected to ythe 4center-tap Bil of the secondarylrf The output of the tube 48fwh-ich1amplies-the second :electricalfsignals is Sconnected -via resistor 98' toi the coil' IIBIJvv which'is' the .primary iof" the y trar-isforiner.' 95. Hencefit will`beapparent that the second.- electricalfsignals"are-fapplied.Sto the coil 04 and the first electrical signals,so long as they are in exact synchronism with the second electrical signals, Will be applied to the coil 78. The circuit including these coils 94 and 78 and the rectiilers 86 and 88 constitutes an important portion of the alarm control circuit F.

It has already been noted that the outside of theV left hand rectier 88 is connected to the tap |34 of D. C. voltage. The right hand end of the other rectifier 86 is connected via resistor |02 and shunt capacitor |04 to the grid |06 of the amplification tube |08. The average voltage applied to the grid |06 will therefore be dependent upon the magnitude and phase relations between the voltages induced in the coils 18 and 94.

In order to explain the functioning of this circuit, let it be assumed that the voltage generated in the coil 18 is 5 volts from the center-tap 80 to each extremity of the coil and that the voltage in the coil 94 is 7 volts. Let it also be assumed that the first and second signals are in exact synchronism. Since each signal is in the form of a discrete surge, preferably of sawtooth shape, for each surge in the primaries .'14 and an alternating current will be generated. in the secondaries 18 and 94 respectively. During the first portion of the cycle of alternation, let it be assumed that the currents generated in both secondaries 18 and 94 fiow to the left as viewed in Fig. 2. The current from secondary 94 corresponding to the second electrical signal will flow into the center-tap 80 and thence will divide, flowing both to th-e left and the right in the coil 78. In the left hand part of'the coil 18 the 5 volts from coil i8 and the 7 volts from coil 94 will add and the current will flow to rectifier 88 which is so oriented as to permit this flow. The rectifier 88 may be consid-ered as a perfect conductor and hence there will be no voltage drop there-across. In the right hand part of the coil T8, the volts generated by the primary 14 will be oriented in a direction against that permitted by therectiiier 86. However, the 7 volts generated in the coil 94 will overpower those 5 volts and will be oriented in a direction permitted by the rectifier 86 so that current will pass through said rectifier, Hence, during this rst portion of the cycle, there will be no voltage drop across either of the rectifiers 86 or 88.

In the next portion of the cycle, currents generated in the coils 'I8 and 94 will be flowing toward the right in Fig. 2. This means that insofar as the current flowing into the coil 94 via the center tap 80 is concerned, it will move to a the left in the right hand side of the coil 18 and to the right in the left hand side thereof. Hence, the voltage in the left hand side of the coil 18 will include 5 volts generated therein by the primary 'I4 and 7 volts corresponding to the voltage of the coil 94, both of these voltages being oriented in a direction opposite to that permitted by the rectifier 68. There will therefore be a total voltage across the rectifier 88 of l2 volts. In the right hand side of the coil 10, there will be a resultant votlage of 2 Volts in a direction opposite to that permitted by rectifier 86 so that there will be a voltage drop of 2 volts across the rectifier 86. The total voltage drop across the two rectiiiers will consequently be volts and-the average voltage drop across the two rectirlers for the entire cycle, assuming that the two halves of the cycle are of equal duration, will be 5 volts. The resistor |02 and capacitor |04 constitute a filter circuit which averages the potential difference across the rectiers 86 and 88, this average potential difference adding to or subtracting from the D. C. voltage source |34 and thus modifying the voltage applied to the control grid |06 of the tube |08.

computations similar to those just made will indicate that so long as the voltage induced in the coils i8 and 94 are in synchronism, an unbalance will exist in the two halves of the circuit just described so that a given modification of the Voltage of the source 22 is impressed on the grid |86. 4If the phase and frequency relationship between the first and second signals be varied so that the signals are not in exact synchronism, this unbalance will be modified and hence the voltage applied tothe grid |06 will be correspondingly altered.

The output of the tube |08 feeds control coil H8 which, when energized pulls down the armature ||2 of a suitable relay or switch H4, the switch controlling whatever sensible alarm may be desired. The alarm may be audible, such as a bell or buzzer, may be visual, in the form of a light, or may merely control suitable alarm equipment in a remote station. The exact nature of the alarm forms no part of the present invention.

The relay H4 is so connected to the alarm and the tube |08 is so biased that so long as a predetermined unbalance exists in the rectifiercoil circuit, the coil H0 will be energized and the relay i4 will be held in open position so that the alarm will not sound.v As soon as said unbalance varies or disappears, however, the voltage applied to the grid |86 will be such as to bias the tube |08 either to out off or so that it will pass an insufficient amount of current, thereby permitting the relay H4 to close and set off the alarm.

The power supply circuit for the apparatus is illustrated in Fig. 3. The regular power supply G may be derived from a standard volt 60 cycle circuit such as is available in normal power distribution systems. The alternating current passes through fuses H6, which may be of 1 ampere, and through switches H3 which are manually closed whenever it is desired to put the system in operation. From there, one side of the source goes to ground and the other side passes through a rectification circuit |20 and a smoothing choke |22 to the high voltage D. C. tap 22. Interposed between the tap 22 and the A. C. source is a rectifier |26 so oriented as to permit current flow to the tap 22.

The alternating current power source is also connected via circuit dropping rheostat |28 to a low voltage line |38 which leads to D. C. connection points |32, |34 and |36 of differing voltages. Y

The standby power source H comprises a low voltage battery |38 and a high voltage battery |40 connected inseries aiding. The low voltage side |42 of the low voltage battery |38 is connected to armature |44 of switch I. The control coil |46 of the switch I is connected via resistor |43 between the regular voltage source G and ground and is so related to the armature |44 as to maintain the latter out of electrical connection with the contact |50, which is in turn connected to ground, so long as the voltage of the regular source G exceeds a predetermined minimum value.

The high voltage end |52 of the low voltage source |38 is connected via manually operated switch |54 to the low voltage line |30 and is also connected to the low voltage side |56 of the high voltage battery Uit. The high voltage end |58 of the high voltage battery |158 is connected Via manually operated switch it!! and rectier |82 to the high voltage tap 22. The switches i@ and H55 are closed simultaneously with the switches H8.

The operation of the apparatus is as follows: The light source A will flash at a rate determined by the magnitude of the resistor Hi and capacitor 24 and the Voltage source to which it is connected, generating second electrical signals in synchronism with the flashes. The photoelectric cell B receives thev flashes and generates rst electrical signals which are amplied and fed to the control element E. The ampli fied second electrical signals are also fedto the control element E, said control element being so biased as to pass current only when the two signals are in synchronism. The' output of the control element E is amplied and directly compared with the second electrical signals, and if that output and the second electrical signals are in exact synchronism as to frequency and phase, a Voltage will be applied to the control grid |08 which will keep the alarm from sounding. If the first electrical signals generated .by the photoelectric cell VB vand the second electrical signals generated by the flashes. ofthe light source A are not in synchronism, vthe grid |06 will 'become so biased as to cause the alarm to sound.

Hence, if an intruder interrupts the light beam 4, the second electrical signals will continue to be sent to the alarm control circuitbut the nrst electrical signals will beinterrupted. An unbalance will no longer exist in the alarm control circuit F andthe alarm'willsound indicating the presence of an intruder. Ifan intruder, by reason of shining an extremely bright light-.onto the photoelectric cell B or'by any othermeans, should cause the control element E to pass current, the alarm would Still sound because the first and second signals would not be in synchronism. If awould-be intruder should cause a counterfeit light source to illuminate the .photoelectric cell B ata' frequency equal to that of the genuine light source A, even then the alarm wouldsound unlessthe counterfeit and genuine light sources were in exact'phase vsynchronism, a condition impossible to achieve from a practical point -oi view. v

If theregular power source LG should fail, the armatureV |44 of the vswitch I will fall andconnect the low voltage battery .L38 of. the :standby vpower source Hin the circuit and therectierri 62 is conductive in a direction to connect thehigh voltage battery |138 in the circuit so thatthe apparatus Will continue to function. Theirectifier H52 preventsl the regularpower source G, when it is effective, from feeding back into the auxiliary .powerl source H, andthe rectifier |26 prevents the high voltage .standby source Mii from feeding back into the main. powersource G whenever its voltage is greaterthan that of the regular power source Gr'fwhile at the lsame time permitting the regular power vsource Gtosupply the high .voltage tap 22.

- Should both power vsources fail, current Will not pass inthe coil and the alarm will sound. Should the lighty source A iail, there will be no second velectrical signals and the alarmwill sound. -Should the ph'otoelectricV cell B vbecome defective, or shouldany of the circuit' elements become disarranged the alarm would also sound.

. From the above it will be realized that bythe arrangement described,A and by the-method of operation set forth, afphotoelectric detection systemhasbeen devised which is fool-proof, dependable, or" such anature as to be capable of being operated by a standby battery source over a long-period of time, simple oi -construction and maintenance, and exceedinglyr valuable in rintruder detection. While only oneembodiment of this invention has beenhere. disclosed, it will be apparent that the method `and apparatus, and particularly the detailed circuit elements thereof, all be varied widely without departing from the spirit of the invention as dened .in the appended claims.

I claim:

1. A photoelectric detection apparatus comprising 4a light source which flashes intermittently, a photoelectric element positioned to receive said ashes and convert them into rst electrical surges synchronized with said received dashes, a signal circuit connected to said light source .for generating second electrical surges in synchronism with the iiashes of said light source, a control element having an electrical input and 'an -electrical output. connections between said photoelectric elements and said signal circuit on the one hand and said. input on the other hand effective to transmit said irst and second surges additivelyin .surge form to said input, and connections.` between said output Yand an alarm control circuit, said control element being effective to produce an electrical output vto said alarm control `circuit `.in synchronism vwith said first surges' only when said firstand second surges are in synchronism, said alarm control circuit utilizing said electrical output .to control azsensible alarm.

2. The photoelectric. detection apparatus of claim 1,- in which said alarm control circuit comprises a center-tapped coil. inductively connected toone yof said electrical output of said `control element and said signal circuit, the other of said electrical output and. xsaidisignal circuit being c'onductively connected. to the center-tap .ofr said coil, an. unbalance thus vexisting when .said electrical output and said .secondsurges are in synchronism; said Acoil beingA connected'to a switch controlling asensible` alarm .in asense such .that said alarm Will-be prevented from. beingrset oi so long fas said imbalance continues to exist.

3.9I'he` photoelectric detection apparatusy `oi claim' 2, which said. .coilis rinductively connected to. said electrical. `output and is conductively connected tosaid signal circuit.

4. The photoelectric'.detection- .apparatus of claim 1, in which said alarm control circuit is also vconnecteolzto said. signalcircuit and is effective :only whensaid. firstv and .second signals are not in synchronismto'set off a sensible alarm.

-5r A.' photoelectric detection apparatus comprising a: light .source which rflashes intermittently, aphotoelectric element positioned to ref ceive-:said flashesy and. .convert them` into iirst .electrical signals, a signal circuit connected. to said light source for generating second electrical .signals in synchronism With said hashes, a vacuum tube-v having electrodes connected'to said 'photoelectric element and. saidsignal circuit and having an output' circuit, .said tube being normally'biasedto cutoff andsaid bias being Yso related to saidnrst and second signals that only when said rst andsecond signals are in synchronism will-.said tube pass current in its output. vcircuit.:V said. current. being in synchronism Y with and having the same frequency as said iirst signals, and an alarm control circuit connected to said output circuit for utilizing the current iowing therein for setting ofi a sensible-alarm.

6. The photo'electric detection apparatus of claim 5, in which said alarm control circuit comprises a center-tapped coil inductively connected to one of said output circuit and said signal circuit, the other of said output circuit and said signal circuit being conductively connected to the center-tap of said coil, an unbalance thus existing when said current in said output circuit and said second signals are in synchronism, said coil being connected to a switch controlling a sensible alarm in such a way that said alarm will be prevented from being set off so long as said unbalance continues to exist.

7. The photoelectric detec-tion apparatus of claim 6, in which said coil is inductively connected to said output circuit and is conductively connected to said signal circuit.

Y y8. The photoelectric detection apparatus of claim 5, in which said alarm control circuit is also connected to said signal circuit and is elective only when current in said output circuit and said second signal are not in synchronism to set 01T a sensible alarm.

9. A photoelectric detection apparatus comprising a light source which flashes intermittently, a photoelectric element positioned to receive said flashes and convert them into iirst electrical signals in synchronism with said received iiashes, a signal circuit connected to said light source for generating second electrical signals in synchronism with the said flashes of said light source, and an alarm control circuit connected to said signal circuit and to said photoelectric element, said alarm control circuit comprising a centertapped coil inductively connected to one of said signal circuit and said photoelectric element, the other of said signal circuit and said photoelectric element being conductively connected to the center-tap of said coil, an unbalance thus existing when said signals are in synchronism, the extremities of said coil being connected to a pair of rectiiiers in series opposing, a' vacuum tube have a control grid connected to one side of one of said rectiers, and a voltage source connected to the other side of the other of said rectiers, the output electrode of said vacuum tube being connected to a coil adapted to actuate a switch controlling a sensible alarm, whereby, so long as said unbalance continues to exist, the average voltage across said rectiers will bias said tube to one output condition, and when said unbalance ceases to exist, said tube will be biased to a different output condition.

10. A photoelectric detection apparatus comprising a light source which flashes intermittently, a photoelectric element positioned to receive said flashes and convert them into iirst electrical signals in synchronism with said received flashes, a signal circuit connected to said light source for generating second electrical signals in synchronism with the said flashes of said light source, and an alarm control circuit connected to said signal circuit and to said photoelectric element, said alarm control circuit comprising a centertapped coil inductively connected to one of said signal circuit and said photoelectric element, the other of said signal circuit and said photoelectric element being conductively connected to the center-tap of said coil, an unbalance thus existing when said signals are in synchronism, the extremities of said coil being connected to a'pair of rectifiers in series opposing, a vacuum tube having a control grid connected to one side of one of said rectifiers and a voltage source connected to the other side of the other of said rectiers, theoutput electrode of said vacuum tube being connected to a coil adapted to actuate a switch controlling a sensible alarm, whereby, so long as said' unbalance continues to exist, the average voltage across said rectiiiers will bias said tube to a cur-i rent passing condition and when said unbalance ceases to exist, said tube will be biased to cutoff.

11. A photoelectric detection apparatuscom-` prising a light source which ashes intermittently, a photoelectric element positioned to receive said flashes and convert them into rst electrical signals in synchronism with said received flashes, a signal circuit connected to said light source for generating second electrical signals in synchronism with the said flashes of said light source, and an alarm control circuit connected to said signal circuit and to `sai-d photoelectric element, said alarm control circuitcomprising a centertapped coil indu-ctively connected to one of said signal circuit and said photoelectric element, the other of said signal circuit and said photoelectric element being conductively connected to the center-tap of said coil, an unbalance thus existing when said signals are in synchronism, the extremities of said coil being connected to a pair of rectiiiers in series opposing, a vacuum tube having a control grid connected to one side of said rectiers and a voltage source connected to the other side of the other of said rectiers, the output electrode of said vacuum tube being connected to a coil adapted to actuate a switch controlling a sensible alarm, whereby, so long as said unbalance continues to exist, the average voltage across said rectiers Will bias said tube to an output condition such that said switch Will be retained in open position and when said unbalance ceases to exist, said tube will be biased to an output condition such that said switch .Wil1 move to closed position and set off said alarm.

12. rI'he photoelectric detection apparatus of claim 9, in which said signal circuit is conductively connected to said coil and said photoelectric element is inductively connected thereto.

13. The photoelectric4 detection apparatus of claim 10, in which said'signal circuit is conductively connected to sai-d coil and said photoelectric element is inductively connected thereto.

14. Apparatus for photoelectric detection comprising a light source, means for causing said light source to flash intermittently, means for detecting said flashes and converting them into first electrical surges synchronized with said received flashes, an electrical circuit adapted to be connected to an alarm for operating the latter in accordance with the magnitude of the current passing through said circuit, a control element connected in said electrical circuit so as to regulate the passage of current through said circuit, means for applying said first surges to said control element, means for applying second electrical surges to said control element in additive relation tosaid first surges, said second surges being in synchronism with the flahes of said light source, the regulatory action of said control element being sensitive to Ythe magnitudes of said first and second surges, the magnitude of each of said rst and second surges alone being insufcient to actuate said control element suiciently to change the status of said alarm but vtheir magnitudes being such that 'only whenthey are applied to said control element togetherandin 13 synchronism will they actuate said control element suiiciently to change the status of said alarm.

15. Apparatus for photoelectric ldetection comprising a light source, means for causing said light source to iiash intermittently, means for detecting said flashes and converting them into first electrical signals synchronized with said received iiashes, an electrical circuit adapted to be connected to an alarm `for operating the latter in accordance with the magnitude of the current passing through said circuit, a vacuum tube having electrodes connected in said electrical circuit so as to regulate the passage of current through said circuit, means for biasing said tube to cut off, means for applying said first surges to an electrode of said tube, means for applying second electrical signals to an electrode of said -tube in additive relation 4to said rst signals, said second signals Ibeing in synchronism with the flashes 4of said light source, the magnitude of ea-ch of said rst and second signals alone being insufficient to overcome the bias of said tube so as to permit it lto pass current, but their magnitudes being such that only when they are applied to sai-d tube together and in synchronism will they overcome said bias and cause said tube to pass current, thus changing the status of an alarm.

16. Apparatus for photoelectric detection comprising a light source, means for causing said light source to flash intermittently, means for detecting said flashes and converting them into rst electrical surges synchronized with said received flashes, an electrical circuit adapted to be connected to an alarm for operating the latter in accordance with the magnitude of the current passing through said circuit, a vacuum tube having electrodes connected in said electrical circuit so as to regulate the passage of current through said circuit, means for biasing said 4tube to cut oi, means for applying said rst surges to an electrode of said tube, means for applying second electrical surges to an electrode of said tube in additive relation to said first surges, said second surges being in synchronism with the flash-es of said light source, the magnitude of each of said rst and second surges alone being insufcient to overcome said Ibias and permit said tube to pass current but their magnitude being such that only When they are applied to said tube together and in synchronism will they overcome said bias and cause said tube to pass current, thus changing the status of an alarm.

17. In the apparatus of claim 14, means for comparing the current in said electrical circuit with said second electrical surges and means for setting off an alarm when said current and said surges are not in synchronism.

18. In the apparatus of claim 15, means for comparing the current passed by said tube with said second electrical signals, and means for setting ci an alarm when said current and said signals are not in synchronism.

19. In the apparatus of claim 16, means for comparing the current passed by said tube with said second surges, and means for setting oft an alarm when said current and said surges are not in synchronism.

SAMUEL M. BAGNO.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,762,725 Marrison June 10, 1930 1,799,993 Staege Apr. 7, 1931 1,971,191 Lord Aug. 21, 1934 2,044,146 Bernarde June 16, 1936 2,062,274 Rees Nov. 24, 1936 2,080,504 Rab-er May 18, 1937 2,202,050 Mitchell May 28, 1940 2,207,540 I-Ianseil July 9, 1940 2,227,147 Lindsay Dec. 31, 1940 2,302,271 Smith Nov. 17, 1942 2,457,502 Shepherd Dec. 28, 1948 

